This invention relates to mirrors, and particularly to mirrors comprising stacked pairs of transparent dielectric layers capped by a reflective layer of metal, particularly gold.
One type of known mirror, particularly useful for multiple light reflections within semiconductor lasers, comprises a stack of contiguous layers of transparent material where the successive layers alternate between high and low refractive indices of light, e.g., odd numbered layers being of a high refractive index while even numbered layers are of low refractive index. At each interface between layers of different refractive indices, some amount of reflection of a beam of light directed at the mirror occurs, with the remainder of the light beam being transmitted through the stack for partial reflection at successive interfaces. For a mirror designed for use with light of a given frequency, all the layers are a quarter wavelength thick (as measured within the layers), and all the light reflected from successive interfaces is exactly in phase where the reflected light beam exits the front of the mirror. The greater the number of pairs of layers (each pair providing in-phase reflections), the greater is the total amount of light reflected and, conceptually, 100% reflectivity could be reached with a sufficient number of pairs. Some light is absorbed in the layers, however, and a peaking of light reflectivity occurs with increasing numbers of pairs. Substantially 100% reflectivity can be obtained, however (or a reduction in the number of pairs for a given reflectivity), by capping or backing the stack with a low optical loss metal layer such as gold, silver, copper or aluminum.
A problem with the use of these metals, and particularly gold and silver, is that they do not adhere well to known dielectric materials heretofore used in mirror stacks. The present invention significantly overcomes this problem.